The county courthouse in White Plains is set to be hijacked Monday by a monumental trial of a celebrity defendant unaccustomed to hearing the words “slow down.’’

Courthouse wags, fed up with the fuss that has overtaken their quiet lives, snarkily call this case “Driving While Kennedy.’’

Standing trial is Kerry Kennedy, the daughter of slain Sen. Robert F. Kennedy, niece of slain President John F. Kennedy, and former wife of Gov. Cuomo, who these days avoids the legal travails of the mother of his three daughters the way one avoids contagious disease.

Kerry, 54, is the professional human-rights activist who allegedly cracked up her 2008 silver Lexus SUV one July morning in 2012 while driving under the influence of the sleeping pill known by the brand name Ambien. She’s charged with driving while ability impaired by drugs.

A misdemeanor.

How did this crummy case get this far?

“Ambien?’’ someone in the courthouse said this week. “All this over an Ambien?’’

But the manner in which the justice system deals with a member of a clan that’s the closest thing America’s got to a royal family will speak volumes about the value society places on money, power and the surname Kennedy.

“This is an unbelievable situation of someone who should have taken a traffic ticket two years ago,’’ one frustrated law-enforcement source told me this week. He blamed Kerry’s relatives, who’ve flocked to her side.

“The Kennedys are the most intransigent about acknowledging any accountability. They don’t want to admit anything. Anyone else would have looked for a way to end this thing.

“Wouldn’t you?’’

Of course, “Kennedy’’ and “driving’’ are two nouns most people don’t like to see in the same sentence. Back in 1969, Kerry’s uncle, the late Sen. Edward “Ted’’ Kennedy, drove his car into the drink off Chappaquiddick Island in Massachusetts, killing his passenger, Mary Jo Kopechne, while Ted swam away. He pleaded guilty to a wrist-slap charge of leaving the scene of an accident causing injury, and served not a minute in jail.

So when Kerry testifies at trial (her lawyer said she will), don’t expect to hear about her unblemished car-pool driving record, but about her efforts to save the world.

She even skipped jury selection in her case this week, traveling to Africa to fight against child slavery and to speak to the European Parliament in Brussels as president of the Robert F. Kennedy Center for Justice and Human Rights.

But helping children may not help Kerry. A judge this month refused to drop the case after Kerry’s lawyers argued that she drugged herself by accident.

On the morning of July 13, 2012, Kerry, dressed in gym clothes, allegedly drove her SUV in and out of traffic on Interstate 684, narrowly missing other cars before slamming the vehicle into a tractor-trailer truck. She continued driving, but then was found by a state highway patrolman slumped over the steering wheel of her vehicle, barely able to walk, talk or see straight. She told the officer she might have mistakenly taken an Ambien pill instead of her daily thyroid medication.

Luckily, no one was hurt.

Then, Kerry changed her tune.

Standing outside the tiny Justice Court in the town of North Castle four days later — the case has since moved to roomier digs in White Plains — Kerry announced that her doctors believed she suffered a “complex partial seizure’’ during the accident that was brought on by an earlier head injury.

A what?

One source said Kerry blamed a car accident for the brain blip. Another told The Post at the time that Kerry might have hit her head while bungee-jumping out of her dormitory window at Brown University more than three decades earlier.

But then, toxicology reports revealed that Kerry had in fact ingested a small amount of zolpidem tartrate, the generic name for Ambien. Accidents and bungee-jumping were quickly forgotten.

So is Kerry Kennedy the victim of a justice system that’s out to nab a Kennedy? Or is she an entitled celeb trying to avoid punishment?

I’d just like to see this case end with Kerry hiring a driver.

A tardy mark for Education Dept.

Fur coat-wearing Marcella Sills, the notorious principal of PS 106 in Far Rockaway, Queens, on Tuesday was removed from her school. A scathing report from Special Commissioner of Investigation Richard Condon confirmed what Post readers knew: The incompetent educator routinely showed up late or never came to school at all while spending taxpayer money on meals and office renovations as kids lacked basic needs like textbooks. Now, officials from the Department of Education said she’ll be fired.

But teachers at 106 have written letters of complaint to education brass virtually from the moment Sills came on board in 2005, one asking to get rid of the “lunatic.’’ One letter was even sent to Condon!

When will the Department of Ed get a clue?

Wary for Carrie

In the span of a week, a Post reporter repeatedly saw Carrie Fisher, the actress who rocketed into teen fantasies as Princess Leia in “Star Wars’’ — a role she’s reprising in the franchise’s seventh installment — drive to a shabby Hollywood apartment building. There, women slipped into the passenger seat of her car for 30 seconds at a time, or drove with Fisher around the block.

The actress was seen furiously bobbing her head up and down, clenching her fists and wiping her nose while looking into a rear-view mirror.

“No, darling,’’ Fisher, who’s been open about her past drug abuse, said when asked if she’d returned to the dark side of the Force.

I’m worried about her.

Scalia knows how to slice it

I’m sure he eats with his hands. While speaking in Chicago, conservative Supreme Court Justice Antonin Scalia, an Italian-American who grew up in Queens, declared something that every New Yorker knows: Gooey Chicago-style deep-dish pizza may be tasty, but it “shouldn’t be called pizza.’’

Mayor de Blasio, a liberal who grew up in Massachusetts, outraged mozzarella lovers by digging into a pie last month with a knife and fork.

Connoisseurs of New York’s thin-crusted delicacy deserve someone who knows his slices.

‘Choice’ words

The good news: Fewer New York City women and teen-age girls than ever are choosing to have abortions, the city Health Department revealed. Just 37 percent of pregnancies were terminated in 2012, lower than at any time since abortion became legal in New York state in 1970.

The bad news: The rate is still double the national average.

I support abortion rights, but I think every terminated pregnancy is a tragedy. The slow-down is a blessing.